Rectifier and method of making it



July 15 1952 J. H. vscAFF :TAL 2,6%,692

RECTIFIER AND METHOD OF MAKING IT Original Filed Dec. 29, 1945 3.3 nrr/E Low EAG/r VOLTAGE n TYPE H/GH BACK VOLTAGE www J. H. scAFF www H.c. THEUERER A TTORNEV Patented July 15, 1952 .lack H. Scai, Summit, N;

.1., and Henry C.

Theuerer, New York, N. Y., assignors to Bell Telephone Laboratories,Incorporated, New

York, N. Y., a corporation of New York .Original'application December29, 1945, Serial No. 638,351. Divided and this application May 22,

1948, Serial No. 28,707

1 Claim. (Cl. 175--366) This application isa division of applicationSerial No. 638,351, filed December 29, 1945, for Rectifier and Methodvof Making It.

This invention relates to devices that conduct electric current morereadily in one directionthan in the opposite direction and to methodsand means of making such devices. it relates more particularly to` suchdeviceswhich include a .body of germanium material.

Electronic asymmetric conductors of electricity may be divided into twogeneral classes, i. e., those in which contact is made between bodies ofdiierent electrical conductivity (l) over a relatively wide area or (2)at a point or several discrete points. In either case there is aboundary condition between the bodies that inhibits current passing inone direction more than inthe other. This invention is concerned withboth types of conductors, but deals more in detail with the pointcontact type. In devices of the point contact type,- a point, usually ofa metallicconductor, is pressed against a surface of a body ofsemiconductive material. Such devices have been called crystaldetectors, or crystal rectiers and also point contact detectors, orpoint contact rectiers. Y

It has been found that a rectifier having particularly desirablecharacteristics may be made by employing a `metallic point contact to kagermanium body. Germanium suitable for rectiers may be of n-type orp-type. Inra rectifier using n-type material, the greater flow ofcurrent occurswhen the body or crystal is negative with respect to thepoint. Conversely, if the greater flow occurs when the crystal ispositive the crystal materialis said to be of p-type. The n-typematerial has a much higher rectifying capability than the p-type.

It ispan object of this invention to improve the characteristics,particularly vthe electrical characteristics, oi germanium typerectiers.

A further object of thisinvention is the produc-` tion of .a germanium,point-contact rectifier capable of withstanding relatively high voltagesin tthe reverse direction.

One feature of this invention resides in the use of germanium of highpurity containing contrcllel, extremely small amounts of certainimpuritiessuch as arsenic, antimony, phosphorus, or bismuth. Forexample, in one illustrative embodiment ci this invention, arsenic ofthe order of 0.00005 per cent but not more than 0.001 per cent may beutilized. In another illustrative embodiment, antimony oi the order of0.001 per cent but not more than 0.01 per cent 2 may be employed.Comparable percentages of phosphorus or vbismuth may be used. j

' Another feature of the invention involves heat treatment of germaniummaterial under such controlled temperature, time, and environmentalconditions, as to produce superior characteristics for its use inrectier devices. n l

The foregoing feature includes heat treatment that converts n-typegermanium to p-type or p-type to n-type, or more generally a lseries ofheat treatments that will convert either type to the other and reconvertit if desired.

The foregoing and other objects and features of this invention will beunderstood more clearly and fully from the following detaileddescription of exemplary embodiments thereofy .with reference totheaccompanying drawing in which:

Figs.- 1 to k5, inclusive, are conventionalized sections, in accordancewith the accompanying legend, of ingots of germanium materials afterdifferent heat treatments;

Fig. 6 illustrates one form oi an area contact, asymmetric Vconductormade of two types of germanium material, the types being indicated inVaccordance with the adjacent legend;

Fig. 7 illustrates one form of pointccntact rectiiier embodying thisinvention; and,k

li'ig.y 8' illustrates another form oi point-contact rectiiier embodyingthis invention.

The crystals employed in the making oi asymmetric conductors inaccordance with this invention are cut from suitable portions of ingotsof germanium material. prepared from germanium dioxide in the, mannerdescribed in the above-identied parent application. This material maythen be doped with materials which alter its electrical characteristics.The liquefied doped vor undoped ger-l manium material is thenprogressively solidiiied from the bottom upward.

Ingots made in accordance with the foregoing procedure contain germaniummaterial which may be characterized as of three different typesseparated into zones. The three diierent types of germanium material,which arereally only two types, p-type and Vn-type, with the latterarbitrarily divided into a high-back-voltage and a low-baclr-voitagetype, are characterized by certain electrical properties. Theseelectrical properties may be determined by making an electric probe teston a suitably prepared surface of a longitudinal section ci the ingot.An ingot made from germanium material to which 0.1 per cent tin has beenadded before or during melting has The ingots may be` made fromgermanium material processed in' a graphite crucible with no tin addedto the melt,

are found to have zones as illustrated in Fig'. 2.

In this material there is a smaller amount -of p-type material, which isusually segregated into small islands near the bottom of the ingot. The

low-back-voltage n-type material occupies a somewhat smaller zone at thetop than 'is'the case with germanium-tin material. The remainder of theingot is high-back-voltage n-type material. In both ingots, thelow-back-voltage n-type material has a reverse peak voltage of the orderof about I to A50 volts. The high-backvoltage n-type material rangesfrom 5'0 to '60 volts back voltage adjacent the top to 1100 1to 150volts back voltage adjacent the bottom in the germanium-tin ingots andto 100 to 4515 volts back voltage adjacent the ybottom in the germaniumingot treated in a graphite Crucible. The p-'type material, which `doesnot appear to Ibe vas good a rectifying material for some purposes asthe n-type, will withstand back voltages of the order of about volts.

Although the illustrations of Figs. ll 'to 4, inclusive, show sharplines of demarcation between 'Zone`s, this is strictly true only betweenthe pand n-type materials. Here there is an abrupt change from p ton-t'ype across what amounts to a barrier. The n-type -material `near the-p-'type will withstand relatively fhigh-back-voltages such as lthee'Volts previously noted. As the top of the ingot is approached the backvoltage vbecomes lower. The line shown lbetween the two zones of n-typewas arbitrarily picked Vat about5'0 volts back voltage. The ingotcontains -fgrea'ter amounts of impurity as the top'is approached, i. e.,the direction-of cooling, and in some cases there is suinic'ientimpurity near the top to 'give the material a lo'W-back-voltagecharacteristic.'

If fan in'got such `as those shown Fig. 1 is heated to about 800 tov900"'C. and'c'ooled rapidly, e. g. air quenched in about nfteenminutes, most of the high-'back-voltag'e n-'type material, except for fasmall reg-ion adjacent the top of thejingot is changed to p-typematerial as illustrated in Fig. 3.V By subsequently heating the rapidlycooled ingot at from 400 to 700 C. or by slowly cooling :from about 800"C. the p-type germanium may lbe entirely transformed to the`higl'r-:ba'ckvoltage n-type. Thus thegermanium material maybe convertedinto all n-type material'of a high-back-Voltage characteristic `except'for a small portion adjacent Athe Vtop of the ingot as in Fig.4.

-Germanium material containing only a very slight trace of `a donorimpurity such as arsenic, Whichis later discussed, and melted inagraphite Crucible with no tin added, will have all of its n-typematerial of the high-back-voltage type. An ing-ot of such material whentreated to convert whatever p-type material there is to n-type will bealluof high-back-Voltage n-type as illustrated in Fig. 5. If there is ahigher amount of impurity there may be some low-back-voltage materialnear the top.

Since specimens of both tin Vaddition and graphite crucible types ofgermanium material when heated to 900 C. and rapidly cooled havesubstantially the same properties as those treated similarly at 800 C.,the lower temperature may ordinarily be used for reasons of convenience.

In samples 'converted to p-'type b'y rapid cooling from vP00" C. andreheated at lower temperatures, it was found that below 400 C. no sub--stantial changes in the characteristics of the p-ty-pe materials areobtained for periods of treatment up to about four hours; between 450 C.:and 650 C. the p-type germanium is converted slowly 'to strongly-rectifying n-type germanium 'of high-back-voltage characteristic; at500 C. or '600 C. the p-type germanium is nearly all converted to n-typein about four hours, complete transformation *occurring within abouttwenty hours.' The rate of conversion from p-type to high-back-Voltagen-type germanium is dependent 'upon the temperature and appears to bemaximum at about 550 C. At 450 C. or 650 C. the conversion is incompleteeven after twenty hours at these temperatures, whereas at 550 AVC.

conversion is compiete in about four hours. -If an n'type lmaterialingot is Aheat treated atabout '700 C. `the bottom portion only isconverted to p-typ'e leaving n-type material of high-backvoltage abovethe p-type-material. As inl the original ingot there is a vsharp `line,of demarcation between theA p 'and n-type. Thus, a piece of materialcould 'be cut from the high-backvol-ta'ge 1i-type zone andheat treatedto make the portionwhich had been 'at the bottom with respect to 'theingot of .p-typ'e leaving n-type at the top. This material could beused. for making conductive devices such as shown in Fig. `6 and whichare later more fully discussed.

The heat treatment vfor converting one type of germanium to the other iscompletely reversible so that conversion in either direction .may beobtained -atk will -by suitable treatment. For example, theAhigh-back-voltage n-type material obtained by means of 1a previouslydescribed cycle of treatment may be reconverted to p-type by heating to800 and rapidly cooling. Also, if high-back-voltage n-type vgermaniumyis desired, it may be obtained vdirectly from the ingots, a'sillustrated in Figs. 1 and '2 by employing the lower temperature 'heattreatment 'at about 550 C. or by slowly cooling from 800 C'.

"Ifov Adetermine whether heat treatment produceda phase transformationin the germanium material, the crystal structure of p and n-typespecimens 'prepared by heat `treatment of adjacent sections from aningot were given an X-ray examination. The structures were, however,identical, the lines Iobtained being in agreement to those reported inthe literature for the diamond cubic germanium lattice. vSubsequentlyprecision measurements were imade of the lattice constant ofthese samesamples; but again no dinerences were* noted.

It is believed that this 'invention may be understood more fully if someof the Vpossible reasons for the behavior of the germanium materialunder `heat 4treatment are discussed. Semiconductors such as areemployed in dry rectiers and like devices 4have been classied as excesssemiconductors or as decit semiconductors. These 'two types have alsobeen `called electronic semiconductors and hole semiconductors. Thetheory is that certain impurities in a substance upset `the electronicbalanceof the atomic structur'efby the addition or subtraction ofelectrons. The'type 'of conductors where electrons are added is theso-called excess type, and the impurity which gives this type ofconductor is known as a donor impurity. In other cases, the impuritiesabstract enough electrons from the principal substance to give theunbalanced or unstable condition which causes current to flow. This isthe dencit type of semiconductor and the impurity which causesthefdelcitby abstracting electrons is known as an acceptor impurity.

Before further discussion of the effect of the donor and acceptorimpurities, it may be well to note that the vgermanium 'materials underconsideration in many respects behave similarly to precipitationhardening alloys. contain a constituent whose solid solubility increaseswith increasing temperature. 'If such an alloy of a given compositionisthen heated above the solubility temperature, the solid solution maybe retained in a metastable state at room temperature by coolingrapidly. On reheating to a temperature-below the solubility temperature,the unstabley solution decomposes,l precipitating a new phase fromsolutionwith resultant changes of physical and electrical properties. In.the present Ainstance the formation of p-type germanium by rapidcooling may result from the retention of an impurity in solution whileformation of n-type germanium may result from precipitation of thisimpurity fromsolution. The known donor impurities for germanium arearsenic, antimony, phosphorus, and bismuth, or in other words,- themembers of the odd series of the fth periodic-group according toMendeleefl.A It has been found that material considered to beessentially pure germanium con'- tains very small amounts of arsenic andphosphorus. Such Imaterial has been successfully used for makinggermanium crystals for rectifiers of n-type rectification. However, whenthese impurities were removed no n-type rectification was obtainedregardless of the treatment 'of the material.v Furthermore, it has beenfound that if proper impurities are added to this material from whichimpuritieshave been removed, n-type rectication can be again obtained.For example, small amounts ofarsenic, antimony, and bismuth have beenadded Vto such materials with satisfactory results. In 4view of theforegoing it seems reasonable to believe that the donor impuritiespreviouslynoted are responsible for. n-type rectification of germanium.It seems probable that an acceptor impurity exists which tend to producep-type rectification. In the region of the ingot which/is ntype, thedonors are in excess and in the region of the ingot which is p-type,acceptors are in excess.-

To explain the inversion in the rectification of germanium which occurson heat treatment, it is necessary to assume that either the donors orthe acceptors can be thermally activated. Assuming that the acceptorsare activated by heat treatment at 800 C.,and are retained in this stateby rapid cooling to lroom temperature, then those portions of the ingotwhich have a higher concentration of active acceptors than of donors,will have p-type rectification.

Because of diierences in the rate of segregation of two impurities onsolidifying an ingot from the bottom upward, the relative concentrationof the impurities may vary at progressive -locations in the ingot. Forexample, the donor may have a lower concentration at the bottom of theingot than the acceptor, and still Suchv alloysy be in excess of theacceptor higher in the ingot. Under; these conditions an ingot, heattreated as above, may have p-type rectification near the bottom Wherethe acceptor is in excess, and n-type rectication at locations higher inthe ingot where the donor is in excess. The line of` demarcationybetween p and n-types is sharply defined. This affords an explanationfor the shell of p-type germanium found in certain' initial ingotsbefore fur-ther heat treatment. Evidently the cooling conditions, whichoccur in the normal cooling cycle, `are such that an excess of activeacceptor is present inthe first material to freeze and the material isp-type. Due to differencesin segregation rates, as freezing progresses,the Vdonor-concentration rapidly overtakes the active acceptorconcentration and an inversion to n-type germanium occurs. If after the800 C; heat treatment, the ingot is subsequently heated at about 500 C'.the acceptors Vare `deactivated and in consequence the donor impuritiesare in virtual excess throughout the ingot and only' n-typerectification is observed.y 1

, An alternative explanation of the heat treatment phenomena involvesthe assumption that the-'donors are deactivated by appropriatethermaltreatment. The reasoning is ranalogous in the former case exceptthat now the 800 C. treatmentdeactivates the donors and rapid coolingretains their inactive form, While subsequent heating atr about 500 C.results in conversion to the active form. To explain complete con-Version to n-typey germanium by the 500 treatment, it is now necessaryto postulate that the active donor concentration is everywhere in excessofthe acceptors. Sincel in some cases the 800 C. treatment results inonly a partial conversion to p-type germanium, it is necessary tosuppose that at high concentrations the donors are incompletelydeactivated. 4

AAlthough both of these explanations are in agreement with theexperimental evidence, the concept of thermally deactivated acceptors ispreferred, because it is compatible with the solid solubility conceptpreviously referred to. In general, it has been observed that impuritieswhich form solid solutions with semiconductors, reduce their resistivityand tend to produce strongly rectifyingmaterials. Since p-typerectification is observed'in ingots rapidly cooled from 800, it seemsreasonable-that the acceptors, which are held in'solid solution by thisprocess arey activated. The` conversion ton-type ygermanium by heatingat500 C'. may then be due to the -deactivation of the acceptors byprecipitation from this'unstable s'olid'solution. l l Although noacceptor impurity hasbeen denitely'identied, it is reasonableto suspectthat this impurity, or one'of them may be oxygen. Thermaltransformations are known to occur in germanium oxide near` 500 C.Moreover, ingots made in graphite crucibles have much less tendency toform p-type germanium, and this may be clue'` to an-initially loweroxygen content in consequence of the reducing nature of the graphite.Furthermore, since the tin in the germanium-.tin composition has beenfound to perform no function in the finished material from theelectrical viewpoint, most of the tin being segre-v gated in thegenerally unusable top portion of the ingot, it seems reasonable tobelieve that tin may also act as a deoxidizer although not to the sameextent as the graphite.

Since the` presence of p-typeV germanium is thought vto be due to anacceptor impurity, such as oxygen, it follows that if this impuritycould be completely eliminated from the ingot, only n-type germaniumwould be found, irrespective cf the thermal history of the ingot. On theother hand, the concentration of the donor impurities might be increasedsuiiciently without changing the acceptor concentration so that an ingothaving only n-type rectication could be produced. This has been found tobe actually the case; but becauseof the higher total impurity content ofsuch ingots, the peak back voltages are reduced to'undesirably lowvalues. For example, note the low-back-voltage vn-type material Yat thetop of the'ingots illustrated in Figs. 1 and :2, which materialis notchanged to p-type (Fig. 8.) nor to high-back-voltage n-type (Fig. 4) by`subsequent .heat treatments. This may be due to additional donorimpurities carried by the tin'wln'ch `was added or t0intentionally-,added impurity.

After processing, the ingot of'germanium material may be cut into smallbodies or crystals for use in rectifiers. Although the preferredmaterial is the n-type high-back-vo'ltage material, :since it mayV beused `to make a rectier of high rectiiication ratio, the other materialwill alsomake a recti'er device. Also, if an ingot such as is shown inFig. '1 be cut soV that a slab or body Acontains both n and p-typematerial an larea-contact or volume type rectiiier such as disclosed inFig. -6 may be made. In this gure, the vslab is made up of a portion '40of high-backvoltage :I1-type material, and a portion 4| of p-typematerial separated by a barrier 46, electrodes v"l2 and 43 are securedrespectively to the two sides of the device, and leads `44 and 45secured, vas by soldering for example, to the respective electrodes.Besides being a rectier the device illustrated in Fig. 6 also exhibitsphotoelectric properties when irradiated at the boundary 46 between thetwo types of germanium at 40 and 4|. As previously noted, an n-type slabmay be suitably rtreated to obtain a conductor like this.

'One form, which the point-contact type of rectifier may take, -is'illustrated in Fig. 7 in which a Amain housing 50 of a ceramic or likeinsulating rmaterial is :provided with metallic end pieces or members Iand 52, which screw'into theopposite ends of the housing 50. Therectifier elements are `carried on the respective vends of pins 53 and.54, tted into bores in the end pieces 5l and 52. metal-coated-on oneside, for example with copper, is secured to the end of the -pin 53,which may be Yof brass, and an S-shaped contact spring 56 vis -securedto 4the end of the-pin 54, which may also -be of brass. The springcontact 56 may be of tungsten suitably pointed at the end which makescontact with the crystal 55. The parts are vadjusted by suitablepositioning of the pins 53 and 54 and are then held in place by meansvof the set screws 51 and '58. The adjustments .are carried on alongwith electrical stabilizing until the device exhibits thecharacteristics desired for a particular purpose. After the adjustmentsare completed, the units are vacuum impregnated with a rsuitable mixturesuch as a wax through the vorifice 59 in the body 5S. Connection may bemade to the reduced portion of the end pieces 5| and 52 by means offriction type connectors 60 and 6|.

A crystal velement 55, which may be is shown in"Fig. 8. A body 75 ofphenolic condensation product or like material has sleeves or cylindersll and l2 molded in opposite ends thereof. Studs 'I3 and 'i4 of brass orlike material, which are a press iit in the sleeves "H and i2, carryrespectively the crystal 'l5 and the springcontact element T6. The studs'I3 and 14 are forced into the sleeves 'H and l2, respectively, inpositions for suitable operation of the device, as determined byappropriate electrical measurements. The studs are then securedin theirrespective sleeves by means of solder as shown at 'll-and'l,respectively. The device 4may be vacuum impregnatedr with a suitable-waxor wax-like mixture through the oriiice 79. Connectors 80 and 8l makinga frictional't with or otherwise secured to' the sleeves 'H and l2,respectively, may be used for making electrical connection to therectifier device.

VIna device such as is shown in Fig. 7 low capacity across the unit is'obtained by means of low dielectric constant insulating vmaterial andsmall diameter of the parts.

CrystalV elements, such as 55 and l5 of the devices shown in Figs. 7 and8, respectively, may be prepared for use before assembly by lapping thesurface to which contact 'is to be made on a suitable smooth surfacewith a fine abrasive. The surface may then be etched. A suitable etchantmay comprise l0 cubic centimeters of nitric acid, 5 cubic centimetershydrofluoric acid and 200 milligrams of copper nitrate lin 10 cubiccentimeters of water. An etching in such a solution forK about thirtyseconds gives a suitable surface@` The active-surface of the crystalelement Vmay also Vbe subjected to an electrolytic etching, whichimproves the device for some purposes by considerably reducing the backcurrent. This etching may be done after the nitric-hydrofluoric etchingpreviously noted, or may be done directly on a lapped crystal withoutthe intermediate etching. Thecrystal may `be etched at a. positiveVpotential of from 4 to 6 volts direct current for from 30 'to 120seconds in 24 per cent hydrofluoric acid.

Although specific Aembodiments of the invention have been shown anddescribed, it will be understood that they are but illustrative and thatvarious modiiications may be made therein without-departing from thescope and spirit of this invention as defined inthe appended claim.

Reference is made to application Serial No. 28,706, filed May 22, 1948,which discloses related subject-matter.

What is claimed is:

An asymmetric conducting device comprising a `housing of low dielectricconstant insulating material `having a central through passage, twocoaxial, thin-walled, metallic, right circular cylindersmolded in saidhousing adjacent opposite ends of the passage and projecting therefrom,two metallic studs, one having a semiconductive crystal and the other ametallic contact point secured to their ends respectively, each studmaking a press fit respectively with one of said cylinders and being soadjusted that the metallic point makes rectifying contact with thesemiconductive crystal, means in addition to the iit between cylindersand studs for securing each stud in adjusted position, and electricalconductors each having a terminal cap coaxial therewith and securedrespectively to the projecting portion of each of said cylinders,whereby the conductors are in line with the studs and cylinders.

JACK H. SCAFF. HENRY C`. THEUERER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date- Hogg July 11, 1905 f Number NameDate Rabezzana June 19, 1934 Masnou June 2, 1936 Jones et a1. Apr. 29,1947 Ohl Mar. 9, 1948 Brattain Mar. 23, 1948 Bieling Apr. 6, 1948 OhlMay 10, 1949

